Production of high purity silicon from amorphous silica

ABSTRACT

A process to the production of silicon from amorphous silica is disclosed. The amorphous silica is formed from a material rich in silica, especially rice husk ash or silica fume. The process comprises subjecting the amorphous silica to leaching with a lixiviant of aqueous mineral acid, especially hydrochloric acid. Preferably, material rich in silica is roasted at a temperature of not more than 850° C., subjected to leaching and then subjected to a second roasting at a temperature of less than 750° C. The process provides for the production of high purity silicon, especially to the production of solar grade silicon (SoG-Si).

FIELD OF THE INVENTION

The present invention relates to a process for the production of highpurity silicon from a material rich in silica, especially from ricehusks or silica fume, and to the production of solar grade silicon(SoG-Si).

BACKGROUND TO THE INVENTION

The search for economical methods to produce silicon for photovoltaicapplications has been ongoing for over three decades. The sources of rawmaterials for these methods have been largely limited to silica in theform of quartz and volatile compounds of silicon, with limitedexamination of rice husk as the source of silicon. The processingmethods that have received the most research attention have been theupgrading of metallurgical grade silicon by modification of conventionalproduction methods and by pyrolytic decomposition of halosilanes i.e.the so-called Siemens Process. These two methods are commonly referredto as the metallurgical and chemical routes to solar grade silicon(SoG-Si) synthesis.

Although the chemical route has been used successfully to meet andexceed the typical purity requirements for SoG-Si, the potential forlowering costs and increasing the volume of production has been verylimited. The unit processing steps involved in the chemical routeapproach can be complex, energy intensive and of long duration, as isdiscussed by Braga A. F. B, Moreira S. P, Zampieri P. R, Bacchin J. M.G, Mei P. R: “New Processes for the Production of Solar gradePolycrystalline Silicon—A Review”, Solar Energy Materials and SolarCells, Vol. 92(2008) pp 418-424.

A metallurgical route combined with directional solidification has beensuccessful in removing metallic impurities with low segregationcoefficients from silicon. However, this route has been ineffectiveeconomically in removing high segregation impurities e.g. B, P, Al. Thelatter are known to be very deleterious to the performance of solarcells, as is discussed in Istratov A. A, Buonassisi T, Pickett M. D,Heuer M, Weber E. R. “Control of impurities in “dirty” multicrystallinesilicon for solar cells” Materials Science and Engineering B 134(2006)Elsevier B.V. pp 282-286. Thus, the logical and consequently typicalapproach has been to use very high purity silica and reductant rawmaterials that are almost free of high segregation impurities.

Rice husk is a waste by-product of the rice milling industry. The ricehusk constitutes about 20% by weight of a rice paddy. The mineral ashcontent is 15-30% of the rice husks, of which 87-97% is amorphoussilica. An estimated eighty million (80×10⁶) metric tonnes of rice huskare generated worldwide annually. The economic potential of utilizingthis large amount of waste rice husk has attracted several researchinterests in recent years, including processing rice husk into othervalue added products e.g. as purified silica, carbides and nitrides ofsilicon, and insulating material in the steel industries.

A method of obtaining silicon of 6N (99.9999%) purity by reducing whiterice husk ash with magnesium at a temperature of 800° C. followed byseveral successive acid leaching treatments is reported by Singh Rajvirand Dhindaw B. K, “Production of High Purity Silicon for Use in SolarCells” Sun, Mankind's Future Source of Energy. edited by Francis deWinter and Michael Cox, Vol. II (1978), pp 776-781, Pergamon Press 1978,authored by International Solar Energy Society (ISES). The possibilityof obtaining silicon of similar purity by direct smelting of purifiedamorphous silica with carbonaceous reductants in an electric furnacefollowed by leaching with acids, and repeating the steps about ninetimes, was also suggested. The method used to analyze the 6N silicon wasnot reported. However, the cost of such repeated smelting and leachingwould be expected to prohibit use of this method as a low costalternative to conventional methods.

U.S. Pat. No. 4,214,920 of Amick et al describes a method for producinghigh purity silicon from rice husks by coking leached rice hulls,adjusting the carbon: silica ratio to 2:1 followed by thermal reduction.Hunt, L. P. Dismukes, J. P. Amick, J. A. Schei, A. and Larsen, K. “RiceHulls as a Raw Material for Producing Silicon” J. Electrochem. Soc. 131,No. 7 (1984) pp 1683-1686 investigated the possibility of producing highpurity silicon from rice husk by purifying rice husk silica according tothe above method of Amick et al, followed by pelletizing and reductionin a modified electric arc furnace. The pelletizing was carried outusing carbon black as a reductant and sucrose as a binder. Modifying theelectric arc furnace was essential to minimize or eliminate possiblecontamination from the furnace atmosphere.

Bose D. N, Govindacharyulu P. A, Barnejee H. D “Large GrainPolycrystalline Silicon from Rice husk, Solar Energy Materials, Vol. 7(1982) North Holland Publishing Company pp 319-321 subjected powderedsilicon obtained by magnesium reduction of rice husk ash to melting anddirectional solidification. It was found that boron was the activeimpurity in the polycrystalline silicon ingot that was obtained. It wasalso determined that the minority carrier life time of thepolycrystalline silicon material was of the order of 1-5 μs, and thuspromising for photovoltaic applications. However, it has beensubsequently estimated that the minimum carrier lifetime requirement forefficient solar cells fabricated from multicrystalline silicon wafers is25 μs. The formation of crystalline silicon by heating a siliconprecursor e.g. silicon dioxide, with an ingredient that will generate anexothermic reaction when heated e.g. magnesium, and isolatingcrystalline silicon is described in US published application2009/0010833 of Rosenband V. et al, published 8 Jan. 2009.

The magnesium reduction of rice husk ash has also been reported byBanerjee H. D, Sen S., Acharya H. N: “Investigations on the Productionof Silicon from Rice Husk by the Magnesium Method”, Materials Scienceand Engineering, 52 (1982) pp 173-179. Acid leached rice husk ash wasreduced by a method involving intimately mixing the ash with magnesiumpowder and firing the powdered mixture at temperatures between 500-600°C. in a sealed graphite crucible in a muffle furnace. The reactionproduct was successively leached in mineral acids (HCl, H₂SO₄, and HF)in a Teflon™ beaker. Some degree of crystallinity in the mufflefurnace-fired rice husk silica was reported. Spectrochemical analysis ofthe final silicon product showed high contents of boron (20-200 ppm),magnesium (50-1000 ppm) and aluminum (10-200 ppm). The contamination ofthe silicon was attributed to the use of laboratory grade magnesium andlaboratory glassware. Nazma Ikram, and Akhter M, “XRD Analysis ofSilicon Prepared from Rice Husk Ash”, Journal of Materials Science, vol23 (1988), pp 2379-2381 reported a similar approach but using 4N puritymagnesium; the silicon obtained was of 99.95% purity with a boroncontent of approximately 2 ppm. It was concluded that the silicon couldbe upgraded to solar grade silicon by conventional refining methods.

Calciothermic reduction of purified rice husk ash was reported by MishraP, Chakraverty A., Banerjee H. D; Production and Purification of Siliconby Calcium Reduction of Rice Husk White Ash, Journal of MaterialsScience, vol 20 (1985) pp 4387-4391. A stoichiometric composition ofgranular calcium and purified rice husk silica was mixed, and thepowdered mixture obtained was fired in a sealed sillimanite crucible ina muffle furnace at a temperature of about 720° C. The reduction productwas milled to fine powder and successively leached with concentratednitric acid (HNO₃) and hydrofluoric acid (HF) to obtain silicon of 99.9%purity with boron content of 10 ppm. It was suggested that the use ofMgO-coated crucibles and high purity reagent could lead to theproduction of solar grade silicon by this method.

Silica fume is a byproduct of the silicon and ferrosilicon productionindustry. It is high purity silica (+90%) in the amorphous form. Use ofsilica fume as the feedstock for solar grade silicon synthesis has notbeen reported.

With respect to operation of economical commercial-scale processes forthe production of solar grade silicon, it is believed that improvementsare required in the methods described above. For instance, combustion ofrice husk has been generally carried out in reactors with hightemperature and long residence time, which results in formation ofcrystalline silica in the ash, which is a known carcinogen. Therefore,treatment or use of the ash from the conventional processes is limited.

A process for the synthesis of high purity silicon, including so-calledSoG-Si, with high volume of production at low cost would be desirable.

SUMMARY OF THE INVENTION

The present invention provides an alternative method for the productionof silicon that has high purity.

Accordingly, the present invention provides a process to the productionof silicon from an amorphous silica, said amorphous silica having beenformed from a material rich in silica, said amorphous silica beingwettable, comprising the step of subjecting the amorphous silica to apurification step in which the amorphous silica is leached with alixiviant of aqueous mineral acid. In embodiments, the amorphous silicahas a loss of ignition content of less than 0.6% by weight.

In a preferred embodiment of the process of the present invention, themineral acid is selected from the group consisting of at least one ofhydrochloric acid, sulphuric acid, acetic acid, citric acid,hydrofluoric acid, nitric acid and perchloric acid. Preferably, themineral acid is a mixture of hydrochloric acid and acetic acid.

In another embodiment, the amorphous silica has been formed by roastingof the material rich in silica at a temperature of not more than 850° C.Preferably, the amorphous silica is rice husk ash. In particular, theamorphous silica is rice husk ash, said rice husk ash having been formedby roasting of rice husks at a temperature of not more than 830° C. Theamorphous silica may also be silica fume.

In a further embodiment, the mineral acid contains at least one metalchloride selected from the group consisting of alkaline earth metalchlorides and ferric chloride. The preferred alkaline earth metalchloride is magnesium chloride.

In yet another embodiment, the amorphous silica has a loss on ignitioncontent of less than 0.2% by weight.

The present invention also provides a process for the production ofsilicon from a material rich in silica comprising the steps of:

-   -   a) subjecting the material rich in silica to roasting at a        temperature of not more than 850° C. for a first period of time,        said temperature and first period of time being selected so that        silica so obtained is amorphous;    -   b) subjecting the amorphous silica obtained in step a) to a        purification step in which the amorphous silica is leached with        a lixiviant of aqueous mineral acid; and    -   c) subjecting the leached amorphous silica obtained in step b)        to a second roasting at a temperature of less than 750° C. for a        second period of time;        step a) preferably being controlled so that the amorphous silica        so obtained has a loss on ignition (LOI) of less than 0.6% by        weight and so that the amorphous silica is wettable.

In a preferred embodiment, the material rich in silica is rice husk orsilica fume.

In another preferred embodiment, the amorphous silica subjected to theleach of step b) has a loss on ignition of less than 0.2% by weight.

In yet another embodiment, the amorphous silica obtained from step c) issubjected to metallothermic or carbothermic reduction to formcrystalline silicon. Preferably, the metallothermic reduction is carriedout in the presence of at least one metal selected from the groupconsisting of magnesium, calcium, and aluminum. Magnesium is preferred.In a preferred embodiment, the metallothermic reduction is carried outat a temperature sufficient to enable exothermic reaction between theamorphous silica and the reductant metal. The preferred temperature isgreater than 550° C. Preferably, the carbothermic reduction is carriedout in the presence of at least one selected from consisting of carbonblack, graphite, char, coal, coke, and residual biomass in rice husk. Inpreferred embodiments, the carbothermic reduction is carried out attemperatures above 1600° C.

In a further embodiment, the metallothermic reduction is carried out inthe presence of inert gas selected from the group consisting of argonand helium, especially argon.

In another preferred embodiment, the reduced amorphous silica soobtained from metallothermic reduction is subjected to at least oneadditional leaching step to purify the silicon product obtained.

In a further preferred embodiment, the additional leaching step isconducted using at least one acid selected from the group consisting ofhydrochloric acid, sulphuric acid, acetic acid, citric acid,hydrofluoric acid, nitric acid and perchloric acid.

In a further preferred embodiment, the leach residue is melted,preferably in the presence of a slag at temperatures above 1410° C. Inparticular, the slag contains at least one of the group CaO, Al₂O₃,SiO₂, MgO, CaF₂, and BaO.

DETAILED DESCRIPTION OF THE INVENTION

The invention is directed to the production of silicon, especially highpurity silicon from a material rich in silica, and in particular frommaterials with silica as the major constituent, such as rice husks,silica fume and fumed silica. In embodiments, the high purity siliconobtained is solar grade silicon (SoG-Si). The preferred material rich insilicon is rice husks, and therefore the invention will be particularlydescribed herein with reference to the production of silicon, especiallyhigh purity silicon, from rice husks. However, it is to be understoodthat the process is also applicable to production of silicon from othermaterials rich in silica.

The present invention provides a process to the production of siliconfrom an amorphous silica, especially silica obtained from rice husks.The rice husks, in the form of rice husk ash, are subjected to aleaching step using a mineral acid. Subsequently, the leached rice huskash is subjected to a reduction by a metal, alloy, or carbon-bearingmaterial, e.g. metallothermic or carbothermic reduction, to convertsilica to silicon. The silicon thus obtained may be subjected to furtherprocessing steps for further purification. Rice husk ash is obtainedfrom rice husks by roasting; in a preferred embodiment, the leached ricehusk ash may be subjected to a further roasting step prior to reduction.

It is understood that silica fume may be used in the process. In such acase, it is not necessary to subject the silica fume to a roasting stepprior to leaching, as silica fume is in the form of the oxide. Moreover,the silica present in industrial by-products such as silica fume may bein the form of amorphous silica.

To obtain rice husk ash, raw rice husk is subjected to high temperature(primary) roasting of rice husks at a temperature of not more than 850°C. The period of time at the high temperature may be varied but istypically from a few seconds to about one minute. It is preferred thatthe rice husks be agitated for uniformity of temperature during theprimary roasting. The silica suitable for leaching in the leaching stepis in the form of amorphous silica, not crystalline silica. Attemperatures above about 850° C., and especially above about 900° C.,silica tends to crystallize or become fused, resulting in silica that isnot amorphous silica. The resultant silica has poorer wettability. Inaddition, crystalline silicates may be health hazards. The time ofheating at temperatures of not more than 850° C., and the temperature,should be controlled to prevent formation of crystallized or fusedsilica. It is preferred that the upper limit of temperature be not morethan 830° C. and especially not more than 800° C. The use of toroidalbed or fluidized bed reactors is believed to be beneficial in theroasting step, in order to obtain a more uniform amorphous silicaproduct. The vessels in any roasting step herein should be low incontent of leachable impurities, especially phosphorus, boron andaluminum, to avoid contamination of the amorphous silica.

In preferred embodiments of the invention, the rice husk ash subjectedto the process of the present invention has a loss on ignition (LOI)content of less than about 0.6% by weight, and preferably less than 0.2%by weight. The LOI content is primarily in the form of carbon, suchcarbon typically being formed during conversion of rice husks into ricehusk ash. In addition to the LOI, rice husk ash contains a wide varietyof impurities, particularly including phosphorus, boron and aluminum.Such impurities are well known to cause deleterious effects in manyend-uses, including in solar cells and other electronic end-uses.

In the process of the invention, the silica-rich phase being rice huskash (RHA) or silica fume is subjected to a leaching step with an aqueousmineral acid. In the leach, the aqueous mineral acid leaches impuritiesfrom the amorphous silica. In order to obtain good surface contactbetween the mineral acid and the amorphous silica, and hence effectiveleaching of impurities out of the amorphous silica, the amorphous silicaneeds to be wettable. Controlled roasting with agitation is believed toprovide wettable amorphous silica. A low LOI is believed to bebeneficial to leaching of impurities in the amorphous silica. Incontrast, a high LOI is believed to be detrimental to wettability,resulting in retention of impurities in the amorphous silica. Highsurface area facilitates wettability, and it is preferred that theamorphous silica have a high porosity.

Examples of the aqueous mineral acid used in the leach include at leastone of hydrochloric acid, sulphuric acid, acetic acid, citric acid,hydrofluoric acid, nitric acid and perchloric acid. The preferredmineral acids are hydrochloric acid and combinations of hydrochloricacid and acetic acid; acetic acid is believed to improve wettability ofthe amorphous silica. In preferred embodiments of the invention, themineral acid contains at least one metal chloride selected from thegroup consisting of alkaline earth metal chlorides and ferric chloride.Examples of the alkaline earth metal chloride are magnesium chloride andcalcium chloride. The preferred metal chloride is magnesium chloride.

The leaching or roasting-leaching steps are followed by metallothermicor carbothermic reduction to form an polycrystalline silicon product.The reduction is carried out in the presence of at least one reductantselected from the group consisting of magnesium, calcium, aluminum,carbon black, graphite, coal, coke, char and residual biomass.Metallothermic reduction, especially using magnesium, is understood tobe preferential compared to use of carbon for lower reductiontemperature, while carbon-bearing materials are preferred for economicreasons. In addition, use of magnesium instead of calcium or aluminum isbelieved to result in a more limited number of unwanted by-product orphases being formed. This facilitates subsequent separation of siliconby known hydrometallurgical processing steps. The product subjected tothe reduction is conveniently in the form of pellets or briquettes,which may be pre-heated to a temperature above about 550° C. Inreduction by carbon, temperature may need to be raised above 1600° C.

The metallothermic reduction of the amorphous silica is preferablycarried out in the presence of inert gas, especially an inert gasselected from argon and helium. High purity argon is preferred.

The reduced amorphous silica from metallothermic reduction, which issilicon, is preferably subjected to at least one additional leachingstep to purify the silicon product obtained. In particular, theadditional leaching step is conducted using at least one acid selectedfrom hydrochloric acid, sulphuric acid, acetic acid, citric acid,hydrofluoric acid, nitric acid and perchloric acid. Hydrochloric acid ormixtures of hydrochloric acid and acetic acid are preferred.

In an embodiment of the invention, the rice husks are subjected to asecondary roasting; this secondary roasting is carried out on leachedamorphous silica and prior to reduction. The secondary roasting stage iscarried out at a temperature of not more than about 750° C. for a periodof time, for example from 30 minutes to several hours. Similar to theprimary roasting, agitation is preferred. It is understood thatadditional stages of roasting could be used in conjunction with theprocessing stages disclosed herein.

In a particularly preferred embodiment of the invention, rice husks aresubjected, in sequence, to a primary roasting step, a leaching step anda further (secondary) roasting step. In particular, the processcomprises subjecting the rice husks to roasting at a temperature of notmore than 850° C. for a first period of time, said temperature and firstperiod of time being selected so that silica obtained is amorphous. Theamorphous silica obtained is subjected to a purification step in whichthe amorphous silica is leached with a lixiviant of aqueous mineralacid. Both of these steps have been described previously. Prior tosubjecting the leached rice husks to reduction, the rice husks aresubjected to a second roasting at a temperature of not more than 750° C.for a second period of time. This roasting stage is carried out at atemperature of not more than about 750° C. for a period of time, forexample from 30 minutes to several hours. In embodiments, the leachresidue is melted in the presence of a slag at temperatures above 1410°C., the slag containing at least one agent of the group CaO, Al₂O₃,SiO₂, MgO, CaF₂, and BaO.

The sequence of primary roasting, leaching and secondary roasting isbelieved to permit improved control of removal of impurities from theamorphous silica, and permit improved removal of impurities in ametallothermic reduction step. Such a step is operated subsequent to thesecondary roasting step.

The present invention provides an improved method for the production ofsilicon, especially by the purification of rice husk or silica fume. Theinvention utilizes a natural source of material or an industrialby-product, rich in silica for the production of silicon, which hasenvironmental and economical advantages. SoG-Si may be obtained.

The present invention is illustrated by the following examples.

EXAMPLE I

Combustion of the rice husk obtained from India was carried out in apilot plant scale toroidal bed reactor at a temperature in the range of800-830° C. to yield rice husk ash with mean particle size of 35 μm andBET surface area 39 m²/g. The ash product was examined under scanningelectron microscope and showed porous particle morphology; the porosityis believed to be due to the continuous toroidal motion (agitation)during combustion. The porous particles responded well to purificationby acid leaching Examination of the ash product under XRD showed acharacteristic amorphous structure in contrast to crystallinity reportedby Banerjee et al (noted above) in muffle furnace-fired rice husk. Thisis believed to be the effect of the combustion method. Visual inspectionof the ash obtained indicated that the distribution of particle size wasnarrow, which might eliminate a need for product sizing.

EXAMPLE II

The ash obtained in Example I was leached with mineral acid(hydrochloric acid) in a non-boron containing vessel (Teflon™container). The rice husk ash was leached with 10 wt % HCl for either 1hour or 4 hours at either 60° C. or 90° C. The leached product wasanalyzed and the results are shown in Table 1. For the leach at 90° C.for 4 hours, it was found that the boron content was reduced from 50 ppmin the unleached ash to <2 ppm; similarly the phosphorus content wasreduced from 1600 ppm to 330 ppm.

TABLE 1 Leached Leached Leached Leached Impuri- Rice husk Ash Ash AshAsh ties ash (800- 60° C./ 60° C./ 90° C./ 90° C./ (ppm) 830° C.) 1 hr 4hrs 1 hr 4 hrs B 50 <2 <2 <2 <2 Al 4000 3021 3088 3091 2645 P 1600 342359 330 340 Cu 11 12 42 100 77 Mn 294 198 202 196 181 Fe 2400 1564 14701499 808 Ni 187 121 374 614 442 Ti 300 337 281 299 265 Zn 30 30 39 37 26Mo 11 6 7 9 5 Cr 68 47 64 97 52 V 3 3 3 3 2 Co 7 4 6 8 6 Na 1700 13301386 1401 1324 K 7900 5147 5343 5280 4915 Ca 3300 1894 1927 1876 1706 Mg1600 973 973 950 885 Ba 149 119 129 132 112

EXAMPLE III

Rice husk ash from Example I was subjected to either (i) leaching at 90°C. for 4 hours followed by roasting, or (ii) roasting followed byleaching at 90° C. for 4 hours. In each instance, the roasting was inair at a temperature of 700° C. with a white-looking ash being obtained.For (i), the carbon content in the ash of Example I was reduced from1.83 wt % to 0.12 wt % and the overall silica content increased from ca.90 wt % to ca. 97%. The analyses of the starting material (rice huskash) and the products of each treatment approach are provided in Table2, showing substantial reduction in the impurities. In particular, thecombination of leaching and roasting, with leaching completed prior tothe roasting, improves the removal of impurities from the amorphoussilica.

TABLE 2 Concentration (ppmw) Sample B Al P Mg Ca Fe Cu Ni Cr K Na RiceHusk Ash 50 4000 1600 1600 3300 2400 11 187 68 7900 1700 Leached &Roasted 0.06 1814 363 856 1130 <2 7 9 29 2757 586 Roasted & Leached 1.512386 406 881 1415 <2 10 40 23 3430 773

EXAMPLE IV

In another series of tests, separate samples (10 grams each) of silicafume collected from dust collection of ferrosilicon production furnaceswere subjected to one of the following procedures: (a) leaching withmineral acid (10 wt % HCl) in a non boron containing vessel for 4 hoursat 90° C., (b) leaching with 10 wt % HCl for 4 hours at 90° C. followedby one hour roasting in a muffle furnace at 700° C., and (c) roasting at700° C. for one hour, followed by leaching with 10 wt % HCl at 90° C.for 4 hours. The analysis of the starting material and the products ofeach treatment approach are provided in Table 3. There was a substantialreduction in the impurities. In particular, the combination of leachingand roasting, with leaching completed prior to the roasting, improvesthe impurity removal.

TABLE 3 Sample B ppm Al ppm Ca ppm K ppm Mg ppm Mn ppm Mo ppm Fe ppm Pppm Silica Fume <2 2153 2527 7265 3004 108 1 1232 294 Process (a) <22094 2026 6149 2585 91 1 1059 <100 (leached) Process (c) <31 1431 575 63222 0 0 984 <125 (leached & roasted) Process (b) <13 1466 1640 3675 3306604 3 7293 <52 (roasted and leached)

EXAMPLE V

To illustrate the reduction process, mixtures of reactants (rice huskash (RHA) and Mg) were compacted into pellets or briquettes using anon-contaminating binder (polyvinyl alcohol solution). The amount of thebinder was 10-15 wt % of the RHA-reductant mixture. The ratio ofreductant (Mg) to RHA was slightly above stoichiometric ratio, with 5 wt% excess reductant. Cylindrical compacts of weight varying between1.4-3.72 g, a height of 5.15-13.45 mm and a compact diameter of 16.3mmwere made using a compression die and under a force of 3-4 tonnes. Thepowdered compacts allowed a higher quantity of material to be charged tothe furnace, compared with a powder mixture, and material losses to thefurnace walls due to passage of inert gas through powder wereeliminated. The reduction of the purified white ash was carried out ininert gas atmosphere (argon).

EXAMPLE VI

In a comparative experiment using a process of the prior art, black ricehusk ash (BRHA) was directly reduced with magnesium turnings of 99%purity, using 25% excess magnesium, in a sealed crucible and mufflefurnace atmosphere. The reduction product obtained was then milled to asize passing −300 μm mesh and leached in HCl at temperature of 70±5° C.for a period of 2 hrs. The black rice husk ash analysed by ICP-MS showeda B and P content of 50 and 1600 ppm respectively and SiO₂ content of90%. The magnesium-reduced black ash analysed by ICP-MS showed a B, Pcontent of <10 ppm (Detection limit of equipment=10 ppm) and P contentof 160 ppm respectively. Leach of the reduction mass with HCl analyzed Band P content of <10 ppm and 20 ppm respectively. A summary of theimpurity reduction by process step is shown in Table 3.

TABLE 3 Concentration (ppmw) Sample B Al P Mg Ca Fe Cu Ni Cr K Na RHA 504000 1600 1600 3300 2400 11 187 68 7900 1700 After Reduction <10 4700160   >25% 1000 1900 212 117 16 800 200 After Leaching <10 2400 20    1.45% 200 700 422 81 10 300 100A general reduction in impurity is observed for all the elements ofinterest except for copper; this apparent contamination was notexplained.

EXAMPLE VII

A mixture of purified white RHA (LOI 0.6%) with magnesium granules (98%purity) at SiO₂/Mg weight ratios of 54.35:45.65 respectively andcorresponding to 5wt % excess magnesium was made into pellets asdescribed above. The pellets were placed in alumina crucible and thecrucible and content placed in the heat zone of a horizontal tubefurnace. The process tube was then flushed with high purity argon at therate of 1 L/min for a period of 10-15 minutes. A heating program wasinitiated after the argon flush with a preferred rate of 300° C./hr. Theargon flow into the process tube during heating was kept to 0.25 L/minHeating was carried to a temperature of 800° C. and maintained at thistemperature for 1 hr after which natural cooling to room temperature wasallowed. XRD analysis of the reduction products showed presence ofsilicon, magnesium silicide (Mg₂Si), MgO, and Mg₂SiO₄.

EXAMPLE VIII

This example shows two-stage leaching.

Stage I Leaching

The reduction mass obtained in Example VII was milled and passed throughTyler Sieve #48 (i.e. passing 300 μm). 5 grams of this sample weretreated with 62.5 mL solution comprising a 4:1 volume ratio of 4M HCland 25% acetic acid respectively. The solid to liquid ratio in thisexample correspond to 8% solids or 80 g/L. Leaching was carried out attemperature of 70° C. for a period of 1 hr. A dry residue of 1.84 g(after a deionized water wash and oven drying at 105° C.) was obtainedwhich was fed to the subsequent leaching process step.

Stage II Leaching

The 1.84 g residual sample obtained in Stage I leaching was treated with40 mL solution comprising a 1:9 volume ratio of 4.8% HF and 25% aceticacid respectively at a temperature of 70° C. for a period of 1 hr. Aresidual 1.64 g of fine grain brownish looking powder was obtained whichwas analyzed by XRD and ICP-OES methods. The concentration of impurityelements is provided below in Table 4.

TABLE 4 3 Stage Leaching Result Concentration (ppmw) Sample B Al P Mg CaFe Cu Ni Cr K Na Silicon Product 26 1540 146 10543 507 561 56 96 1962746 659

EXAMPLE IX

This example show 3-stage leaching.

Stage I Leaching

The reduction mass obtained in Example VII was milled and passed throughTyler Sieve #200 (i.e. passing 75 μm). 5 grams of this sample weretreated with 830 mL solution comprising a 4:1 volume ratio of 4M HCl and25% acetic acid respectively. The solid to liquid ratio in this examplecorrespond to 0.6% solids or 6 g/L. Leaching was carried out attemperature of 70° C. for a period of 1 hr. The dry residue of 1.70 gwas obtained which was fed to the subsequent leaching process step.

Stage II Leaching

The 1.70 g residual sample obtained in Stage I leaching was treated with85 mL solution comprising a 4:1 volume ratio of 2M H₂SO₄ and 25% aceticacid respectively at temperature of 70° C. for a period of 1 hr. Aresidual 1.65 g of fine grain brownish looking powder was obtained whichwas fed to the subsequent leaching process step.

Stage III Leaching

The 1.65 g residual sample obtained in Stage II leaching was treatedwith 82.5 mL solution comprising a 1:9 volume ratio of 4.8% HF and 25%acetic acid respectively at temperature of 70° C. for a period of 1 hr.A residual 1.28 g of fine grain brownish looking powder was obtainedwhich was analyzed by XRD and ICP-OES methods. The concentration ofimpurity elements is provided in Table 5.

TABLE 5 3 Stage Leaching Result Concentration (ppmw) Sample B Al P Mg CaFe Cu Ni Cr K Na Silicon Product 27 1178 156 9813 258 134 27 83 168 2556578

The 1.28 g residue obtained after stage III leaching was in goodagreement with theoretical calculations of the amount of silicon to beobtained based on the initial composition of the reduction mass.Recovery was high. The higher B and P content in the silicon product isbelieved to be due to contamination.

EXAMPLE X

To assess the effect of initial charge composition on the reductionproduct that is formed at a given temperature, the initial magnesiumcontents in the charge were varied at 0, 5, 11.5 and 25 wt % excess; 0%excess corresponds to stoichiometric magnesium-silica composition. Thereduction of each batch sample was carried out at 650° C. under argonatmosphere as outlined in Example VII. The phases present in each of thereduction sample were analyzed by powder XRD technique and thequantitative amount of each phase determined by the Reitveld method. Theresults of the Reitveld quantitative XRD analyses of the samples aresummarized in Table 7. It is seen that the maximum silicon yield ofabout 72% is achieved for a charge with 5 wt % excess magnesium.

TABLE 7 Effect of excess magnesium on phases in reduction product at650° C. % Excess Temp. Phases (wt %) Mg ° C. Si MgO Mg₂Si Mg₂SiO₄ α-SiO₂0 650 15.98 63.55 12.67 7.40 0.40 5 650 18.38 61.08 10.37 9.84 0.33 11.5650 13.00 67.00 13.60 5.60 0.80 25 650 6.60 50.70 30.40 12.20 0.00 MeanError % 0.5 1.0 0.6 0.7 0.1

EXAMPLE XI

To assess the effect of temperature on the reduction product that isformed at a fixed charge composition (5 wt % excess magnesium), thereduction temperatures of batch samples were varied at selectedtemperatures between 500-950° C. under argon atmosphere as outlined inExample VII. The phases present in each of the reduction samples wereanalyzed by powder XRD technique and the quantitative amount of eachphase determined by the Reitveld method. The results of the Reitveldquantitative XRD analyses show that the recovery of silicon in thereduction product increases with temperature with maximum yield of 85%at 900° C.

EXAMPLE XII

The effectiveness of reagent type and concentration on the dissolutionof the magnesium phases during first stage leaching of the reductionproduct having an initial charge composition of 5 wt % excess magnesiumand reduced at 900° C. was evaluated. The reagent types and theireffective concentration that were tested are summarized in Table 8. One(1) gram samples of the homogenized reduction product having totalmagnesium content of 45.7 wt % was leached with 500 ml each of the testreagents listed in Table 8 for a definite time. Aliquots of the leachsolution were taken at fixed time intervals for each test run and thesolution analyzed for the magnesium ion concentration by ICP method. Theresults reveal that reagent type I which is a combination ofhydrochloric and acetic acid with effective hydrogen ion concentrationof 1M was a better lixiviant than the others, giving 99% removal ofmagnesium compounds after 15 minutes.

TABLE 8 Leaching reagent type and effective concentration HCl CH₃COOHReagent Conc. Volume Conc. Volume Effective Type (M) % (M) % Conc. (M) I1.25 80.0 4.38 20.0 1.00 II 1.25 100.0 0.00 0.0 1.25 III 4.00 80.0 4.3820.0 3.20 IV 4.00 100.0 0.00 0.0 4.00

EXAMPLE XIII

A 5 gram and 30 gram reduction product samples having composition of 5wt % excess magnesium content and reduced at 900° C. were each leachedat 6 g/l and 20 g/l respectively using reagent type-1 listed in Table 8at temperature of 70° C. The weight fraction of dry residue retainedafter first stage leaching in each case was approximately 32% of theinitial sample weight. Each of the residues was further subjected to asecond leaching at 20 g/l using a reagent mix of 4.8 wt % HF and 25 wt %CH₃COOH in a volume ratio of 1:9 respectively. The leaching temperaturewas maintained at 70° C. The weight fraction of dry residue retainedafter second stage leaching in each case was approximately 81% of theinitial sample weight. Chemical composition analyses of the two siliconproducts gave the range of impurities summarized in Table 9.

TABLE 9 Range of impurities in as produced silicon from rice huskConcentration (ppmw) Impurity B Al P Mg Ca Fe Cu Ni Cr K Na Range 3-181265-1581 25-73 1078-16266 276-672 534-658 0.1-45 38-57 73-118 1477-2803422-753 (low-high)

EXAMPLE XIV

5 gram silicon sample from the as-produced silicon powder in ExampleXIII containing 18 ppmw B and 73 ppmw P was melted together with a slagfor further refining. The slag, weighing 7.0 grams, was prepared byblending reagent grade oxides to form a 10Al₂O₃:42CaO:10MgO:38SiO₂ (wt%) mixture. The mixture was compacted into two disks and loaded withsilicon into a magnesia crucible. The crucible was heated to 1500° C. ina vertical tube furnace under high purity argon flow. After holding fortwo hours, the crucible was withdrawn and quenched in water. A sample ofsilicon was analyzed and showed P and B levels of 5.5 and 7.5 ppmwrespectively. These represent about 93 and 58% reduction in theconcentrations of P and B respectively.

EXAMPLE XV

Silica fume containing 97% SiO₂, 0.9% K₂O, 0.5% MgO, 0.5% Carbon, 0.4%Al₂O₃, 294 ppmw P, and below 2 ppmw B was processed under the followingconditions to obtain high purity silicon. (a) 100 g of the material wasleached in a heated Teflon™ beaker, in 10 wt % HCl solution for fourhours, while maintaining solid content at 10%, and temperature at 90° C.(b) The residue was rinsed with de-ionized water, and dried, and thenroasted in a muffle furnace under air atmosphere. The temperature of thefurnace was held at 700° C. for four hours. (c) A batch of the purifiedsilica fume was mixed with magnesium granules of 98% purity, at a weightratio of Silica Fume/Mg of 5:4. 2 g of the blended batch was loaded intoan alumina tube, which was inserted inside a steel capsule. The reactioncell was then placed inside a muffle furnace and heated at the rate of5° C./min to 750° C. After 2 hours, the furnace was cooled to roomtemperature. Quantification of the product composition by XRD andRietveld method indicated 90% yield for silicon. (d) The reactionproducts were mildly hand-milled and leached in two steps, first in amixture of 2M HCl solution that also contained 25% acetic acid andsecond in a mixture of HF (4.8%) and acetic acid (25 wt %). The residuewas washed with de-ionized water and dried for two hours at 110° C. ICPanalysis of the silicon product shows the following composition (ppmw);B<3, P 12, Ca 1041, Fe 1203, Al 2255, Ni 40, K 1420, Mg 3637, Cr 9, V 1,Mo 10, Mn 150, Zn 67, As 1, Cd 3.

EXAMPLE XVI

2 gram of the silicon produced under conditions of Example XV wasleached at 95° C. with 10 ml of a magnesium chloride/hydrochloric acidlixiviant in which the ratio of MgCl₂/HCl/H₂O was 3:4:4 by weight. Theleaching time was 240 minutes; the leaching mixture of silicon andlixiviant was stirred at 400 rpm. The residue obtained from the leachwas rinsed with de-ionized water, dried at 110° C. and analyzed usingICP-MS. The analysis showed that the impurities had been reduced to thefollowing levels (in ppmw): B<3, P:,10, Ca: 79, Fe 11, Al22, Ni 11,K,0.01, Mg 23, Cr 2, V 1, Mo 0.05, Mn 1, Zn 0.5, As 0.7, Cd 0.02.

1. A process to the production of silicon from an amorphous silica, saidamorphous silica having been formed from a material rich in silica, saidamorphous silica being wettable, comprising the step of subjecting theamorphous silica to a purification step in which the amorphous silica isleached with a lixiviant of aqueous mineral acid.
 2. The process ofclaim 1 in which the mineral acid is selected from the group consistingof at least one of hydrochloric acid, sulphuric acid, acetic acid,citric acid, hydrofluoric acid, nitric acid and perchloric acid.
 3. Theprocess of claim 2 in which the mineral acid is a mixture ofhydrochloric acid and acetic acid.
 4. The process of of claim 1 in whichthe amorphous silica has been formed by roasting of the material rich insilica at a temperature of not more than 850° C.
 5. The process of ofclaim 1 in which the amorphous silica is rice husk ash or silica fume.6. The process of claim 5 in which the amorphous silica is rice huskash, said rice husk ash having been formed by roasting of rice husks ata temperature of not more than 830° C.
 7. The process of of claim 1 inwhich the mineral acid contains at least one metal chloride selectedfrom the group consisting of alkaline earth metal chlorides and ferricchloride.
 8. The process of claim 7 in which the metal chloride ismagnesium chloride.
 9. The process of of claim 1 in which the amorphoussilica has a loss of ignition content of less than 0.6% by weight. 10.The process of claim 9 in which the amorphous silica has a loss onignition content of less than 0.2% by weight.
 11. A process for theproduction of silicon from a material rich in silica comprising thesteps of: a) subjecting the material rich in silica to roasting at atemperature of not more than 850° C. for a first period of time, saidtemperature and first period of time being selected so that silica soobtained is amorphous; b) subjecting the amorphous silica obtained instep a) to a purification step in which the amorphous silica is leachedwith a lixiviant of aqueous mineral acid; and c) subjecting the leachedamorphous silica obtained in step b) to a second roasting at atemperature of less than 750° C. for a second period of time.
 12. Theprocess of claim 11 in which the amorphous silica obtained from step c)is subjected to metallothermic or carbothermic reduction to formcrystalline silicon.
 13. The process of claim 12 in which the reductionis a metallothermic reduction carried out in the presence of at leastone metal selected from the group consisting of magnesium, calcium, andaluminum.
 14. The process of claim 13 in which the metal is magnesium.15. The process of claim 13 in which the metallothermic reduction iscarried out at a temperature sufficient to enable exothermic reactionbetween the amorphous silica and the reductant metal, said temperaturebeing greater than 550° C.
 16. The process of of claim 13 in which themetallothermic reduction is carried out in the presence of inert gasselected from the group consisting of argon and helium.
 17. The processof claim 12 in which the reduction is a carbothermic reduction carriedout in the presence of at least one agent selected from consisting ofcarbon black, graphite, char, coal, coke, and residual biomass in ricehusk.
 18. The process of claim 17 in which the carbothermic reduction iscarried out at temperatures above 1600° C.
 19. The process of of claim12 in which the reduced amorphous silica so obtained from reduction issubjected to at least one additional leaching step to purify the siliconproduct obtained.
 20. The process of claim 19 in which the additionalleaching step is conducted using at least one acid selected from thegroup consisting of hydrochloric acid, sulphuric acid, acetic acid,citric acid, hydrofluoric acid, nitric acid and perchloric acid.
 21. Theprocess of claim 20 in which residue from the leach of step c) is meltedin the presence of a slag at temperatures above 1410° C., said slagcontaining at least one agent selected from the group consisting of CaO,Al₂O₃, SiO₂, MgO, CaF₂, and BaO.
 22. The process of of claim 11 in whichstep a) is controlled so that the amorphous silica so obtained has aloss on ignition (LOI) of less than 0.6% by weight and so that theamorphous silica is wettable.
 23. The process of claim 22 in which theamorphous silica subjected to the leach of step b) has a loss onignition of less than 0.2% by weight.
 24. The process of of claim 11 inwhich the material rich in silica is rice husk or silica fume.